Physicist and astronomer Karl Schwarzschild developed the concept of black holes, showing that when stars undergo gravitational collapse and shrink below a certain size (now called the Schwarzschild radius) the gravitational pull becomes so great that not even light can escape. He was able to use photographic plates to determine the apparent magnitude of stars, theorized that space is non-Euclidean, determined the diameters of particles in comets' tails, and calculated the first rigorous solution to Albert Einstein's field equations. His sister married physicist Robert Emden. Schwarzschild is the namesake of the German Astronomical Society's Schwarzschild Medal, and his son, astronomer Martin Schwarzschild, was the first recipient of this honor.